LIFE-PROCESSES OF CORAL COLONY 107 



The original nucleus has been covered equally upon all sides, 

 and the weight of the colony is not sufficient to cause the 

 death of those zooids which happen to live on the under side. 

 But as the mass increases in size and weight, death of the 

 lowest zooids must inevitably occur, and the rest of the surface- 

 area carrying on the compensating building will cause the 

 growth to become dome-shaped. Theoretically, the dome 



Fig. 36. 



Fig. 37. 



Young For lies Mass grown 



EQUALLY ROUND A CENTRAL 

 NUCELUS. 



Older Porites Colony in which 



THE Lower Zooids are killed 



BY Pressure. 



shape would be the type of form of all the massive species 

 which follow this method of growth and division, but practically 

 the dome shape is far less common than the flat-topped rock, 

 and this is for the reason that injury to the uppermost zooids 

 is usual in the life-history of a colony. 



When the dome has become of some size, its upper surface 

 becomes large enough and flat enough to form a resting-place 

 for sediment, and the uppermost zooids decline in activity, the 

 compensating growth carries the sides farther out, and the 

 colony tends, by the increase of the rest of the surface, to 

 become still more flattened at the top. Injury caused by 

 loosened fragments sweeping over the surface of the rocks, 

 and the further deposition of sediment, finally cause the 

 wholesale death of the zooids of the flattened tops. The 

 zooids round the margins now form, by their active growth, 



